Method for the protection of metal articles at elevated temperatures

ABSTRACT

A method of protecting an article made of zirconium, titanium, hafnium, niobium, tantalum, vanadium or their alloys against high-temperature corrosion consisting of pickling the surface to be protected, galvanizing an alloy having an aluminum-silicon base and containing the metal which is to be protected and an equivalent surface and brazing the two surfaces together with the aid of a brazing metal selected from the group of copper, silver, beryllium, silicon and their alloys with one another.

United States Patent [72] Inventor Michel Clnrveriat Albertvllle, France [21] Appl. No. 776,812 [22] Filed Nov. 18, 1968 [45] Patented July 27, 1971 [73) Assignee Uglne Kuhlmum Paris, France [54] METHOD FOR THE PROTECTION OF METAL ARTICLES AT ELEVATED TEMPERATURES 7 Claims, No Drawings [52] US. Cl 29/492, 29/497, 29/502, 29/504 (51] Int. Cl B231: 31/02 [50] Field of Search 29/492, 497, 502, 504, 197, 198, 488

[56] References Cited UNITED STATES PATENTS 2,100,258 11/1937 Larson 29/197 2,768,271 10/1956 Meredith..... 29/492 X 2,798,843 7/1957 Slomin et al. 29/198 UX 3,101,531 8/1963 Roseberry 29/502 X 3,106,773 10/1963 Jafie et a1. 29/492 X 3,213,532 10/1965 Glaser et al 29/492 X 3,397,445 8/1968 Ulmer et al 29/502 X 3,417,461 12/1968 Wells et al. 29/504 X 3,442,641 5/1969 Albers 29/497 X FOREIGN PATENTS 809,125 3/1956 Great Britain 29/504 825,317 11/1957 Great Britain... 29/492 939,871 8/1961 Great Britain 29/504 3624.209 9/1961 Japan 29/504 Primary Examinerlohn F. Campbell Assistant ExaminerRonald .1. Shore Attorney-Webb, Burden, Robinson & Webb ABSTRACT: A method of protecting an article made of zirconium, titanium, hafnium, niobium, tantalum, vanadium or METHOD FOR THE PROTECTION OF METAL ARTICLES AT ELEVATED TEMPERATURES My inventionrelates to a process for protecting certain metal articles from corrosion. More particularly, my invention relates to a process for protecting tubular products made from metals such as zirconium, titanium, hafnium, niobium, tantalum, vanadium and their alloys from corrosion at elevated temperatures.

in French Pat. No. 1,456,754, l described a process for protecting the above-mentioned metals. My process in the French Pat. consists of depositing a thin intermetallic layer with the general formula, Al,Si,,Me,, on the surface of the metal to be protected. The process may also provide a superficial layer rich in aluminum. The Me in the above formula represents the metal of the piece that is to be protected.

The combination of this intermettalic layer with the aluminum-rich superficial layer protects the metal against intergranular corrosion and against the diffusion of the aluminum into the metal at elevated temperatures.

To effect this double protection, by French Pat. No. 1,456,764 describes a process in which the article to be protected is plunged into a liquid bath composed of an alloy with an aluminum base containing 1 to 40 percent silicon and saturated with the base metal of the article. The temperature range of the bath is from 575 to 1000" C.

Although this method of protection proves very satisfactory in the case of small articles, it does not allow one to obtain Duplex tubes of large dimensions (by this l mean interiorly coated tubes whose length ranges from 1 to 6 meters and whose diameter ranges from 50 to 200 mm.) due to the difficulties encountered in making furnaces of sufficient size and shape and which will contain liquid aluminum.

Another method for making a protective surface consisting of an intermetallic layer, Al,Si,,Me,, and an outer layer rich in aluminum is described in Belgian Pat. No. 685,480. The Belgian patent is based on the proven fact that a continuous layer of this intermetallic compound can be formed without the necessity of passing the article through a liquid bath.

The Belgian patent teaches covering the article to be protected with a solid alloy having an aluminum and silicon base with a silicon content ranging from 0.1 to percent of the total weight in such a manner that the alloy is pressed firmly against the surface to be protected in order to assure intimate contact therebetween. The entire unit is then raised to a temperature ranging from 380 to 660 C, the melting temperature of aluminum, while maintaining the pressure for applying the alloy against the surface that is to be protected. A continuous and protective layer is formed consisting of an intermetallic compound having the formula Al,Si,,Me,. This layer is of the same type described in my French Pat. No. 1,456,754.

The temperature selected for heating the article and the duration of this heating have an influence on the thickness of the protective layer obtained. Higher temperatures and prolonged heating result in an increase in the thickness of the protective layer. In borderline cases the diffusion barrier can be very thin and even difficult to detect. However, it may appear very clearly after being subjected to heat in the course of its use.

However, the processes such as culmination, coextrusion, codrawing, and metallization with a spray gun, which are described in Belgian Pat. No. 685,480, are very difficult to carry out in the case of very long tubes. in fact, these processes used either with a brazing such as Cu, Ag or Al-Si or used without the aid of brazing have led to irregular results on tubular specimens of large dimensions. For example, thin tubes of Al-Si (1 percent Si) were bonded to the interior of Z ircaloy 2 (a zirconium base metal containing about 1.5 percent Sn, 0.15 percent Fe, 0.10 percent Cr and 0.05 percent Ni) tube. These bonds were accomplished by brazing the Zircaloy 2" to the thin tube. The brazing composition was 92.7 percent A1, 7.0 percent Si and 0.3 percent Zr. The conditions of brazing were either a gaseous atmosphere with the two tubes held together by an internal nitrogen pressure of kgjcm. or a vacuum with an expandable mandrel exerting the necessary pressure. An irregular bonding was obtained that was characterized by the partial separation of the coatings when subjected to tests of thermic cycling between 100 and 400 C. In addition, Al-Si tubes have been bonded without brazing to the interior of Zircaloy 2" tubes with large diameters (diameter=92-98 mm.) and lengths between 200 and 500 mm. These bonds formed under vacuum at 630 C possessed the desired intennetallic compound. However, the tubes separated after a reduced number of thermic cycles.

My invention consists of a process for protecting from hightemperature corrosion'a tubular article made from a metal such as Zr, Ti, Hf, Nb, V or their alloys To show the preferred embodiment of my invention, I will describe the process of protecting the inner surface of the metal tube. It will, of course, be understood that either surface could be protected as could the outer surface .of a solid bar or shape. The initial step consists of pickling the inner surface ofthe tube to be protected by means well known in the art. A second tubecomposed of an Al-Sibase alloy serves as the protecting mechanism. This Al-Si base alloy can also contain the metal which is to be protected. The Al-Si base tube is of a length comparable to the tube to be protected and of a slightly smaller diameter so it can be positioned within the tube to be protected. The Al-Si tube is zinc coated (galvanized) by conventional means such as immersing the tube in an aqueous solution of sodium zincate. A brazing material selected from the group consisting of Cu, Ag, Be, Ge, Si and the alloys of these metals with one another is placed on either the inside of the tube to be protected or the outside of the galvanized Al-Si base tube. The Al-Si tube is then positioned at ambient temperature within the tube to be protected. The diameter of the Al-Si tube is such that it contacts the entire surface of the outer tube. A permanent mechanical pressure necessary for satisfactory cohesion is exerted to force the Al-si tube against the outer tube. This pressure is preferably exerted by an expandable plug or mandrel fitted into the interior of the tube and having a substantially greater coefficient of expansion. The assembly is the raised to a temperature between 380 and 660 C. for a period of at least one hour under a residual pressure equal at the most of 10 torr. The assembly is then cooled and the mechanical pressure is released.

This technique has produced excellent results in tubular specimens of rather large dimensions, andthe following nonlimiting examples given below will illustrate the proposed technique.

EXAMPLE 1 A tube with an inside diameter of mm., a thickness of 3 mm., and made of a zirconium alloy containing 1.5 percent Sn, 0.12 percent Fe, 0.10 percent Cr and 0.05 percent Ni was protected by my invention. The length of this tube was 200 mm. An Al-Si alloy containing 1 percent Si was prepared in the form of a tube with an inside diameter of 88 mm., a thickness of 0.6 mm., and a length equal at least to that of the zirconium tube. The Al-Si tube was then galvanized with a very thin layer by immersion in a aqueous solution of sodium zincate. A layer of copper about 4 microns thick was then deposited on the exterior surface of the Al-Si tube by electrolysis in a bath with a base of sodium cyanide.

The Al-Si alloyed tube was placed into the interior of the zirconium alloy tube and'was cold plated by passing an oliv" with an outside diameter of 89.4 mm. through the interior of the assembly. The passing of the olive" firmly laid the Al-Si tube flat against the inside wall of the zirconium alloy tube. The Al-Si tube actually underwent an elongation of about 10 percent. A cylindrical plug of austenitic steel of the 18/10 type with an outside diameter of 89 mm. was then placed into the interior of the assembly. The assembly was subjected to heat treatment under vacuum for a period of 1 hour at 615 C. 1n the course of this heating the plug expanded more than the outer zirconium alloy tube and kept the Al-Si tube firmly pressed against the zirconium alloy tube. After cooling and removal of the plug, the Al-Si tube adhered firmly to the zirconium alloy tube. A thin layer of an intermctallic Al-Si-Zr compound was interposed between the Al-Si and the zirconium alloy. This layer prevented the diffusion of the aluminum into the zirconium alloy at elevated temperatures, such as EXAMPLE 2 The same tubes were used as those in example 1, but work was done with pieces 500 mm. in length. As in example 1, the outer surface of the Al-Si tube was galvanized, then copper plated.

For the plug a composite alumina-aluminum material with an outside diameter of 89 mm. was used, and the assembly was heated under vacuum for 2 hours at 585 C. After cooling and removal of the plug, the two tubes adhered perfectly to one another with a thin layer of an AlSi-Zr intermetallic compound interposed between them. To prove this adhesion, the unit was subjected to 300 cycles comprising prolonged heating for a half hour at 400 C., followed by rapid chilling. At the end of the 300 thermic cycles of this type no separation was observed.

lclaim:

1. A process for protecting a first metal surface selected from the group consisting of Zr, Ti, Hf, Nb, V and alloys thereof from corrosion at high temperatures bybonding an aluminum-silicon base alloy thereto, said bonding comprising:

A. Pickling the surface of the first metal;

B. Galvanizing a surface of said aluminum-silicon base al- C. Placing on one of said surfaces a brazing material selected from the group consisting of Cu, Ag, Be, Ge, Si and alloys thereof; and

D. Brazing the pickled surface with the galvanized surface.

2. The process of claim I wherein the aluminum-silicon base alloy is mechanically held against said metal surface.

3. The process of claim 1 wherein said aluminum-silicon base alloy includes as an alloying element at least one of the metals of the first group.

4. The process of claim 1 wherein the first metal is an elongated tubular article and said aluminum-silicon base alloy is a conforming tubular article, said conforming tubular article positioned within and in intimate contact with said first tubular article for protecting the interior surface of said first tubular article.

5. The process of claim 4 wherein an expandable plug having a substantially greater coefficient of expansion than said tubular articles is fitted into the interior of and in contact with the interior surface of said conforming tubular article, and heating said tubular articles and said plug, whereby said plug expands to a greater extent than said tubular articles to force said conforming tubular article against the interior surface of said first metal tubular article.

6. The process of claim 5 wherein said heating is from about 380 to 660 C. at a pressure up to 10" torr for a period of at least 1 hour to braze said tubular members.

7. In a process for protecting the interior surface of a first elongated tubular article of a metal selected from the group consisting of Zr, Ti, Hf, Nb, V and alloys thereof from corro sion at high temperatures by brazing the outer surface of a second tubular aluminum-silicon base alloy article to said interior surface with a brazing material selected from the group consisting of Cu, Ag, Be, Ge, Si and alloys thereof; the improvement comprising: galvanizing the outer surface of said second tubular article.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 594, 896 Dated J l 27, 1971 Inventor(s) Michel Charve riat It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 Line 22 -by French Pat. should read --rny French patent--. Column 2 Line 14 --alloys-- should read --alloys. --Column 2 Line 41 the-- should read then-. Column 2 Line 43 10 should read l0 Column 2 Line 65 -olivshoual read --olive- In Claim 6, Column 4 Line 26 -10 should read 10' Signed and sealed this 18th day of January 1972.

(SEAL) Attest:

EDWARD M.FIETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents I FORM Po-1050 [IQ-6 USCOMM DC 60376 P59 U 5 GOVERNMENT PRINTING OFFICE I969 0-366-334 

2. The process of claim 1 wherein the aluminum-silicon base alloy is mechanically held against said metal surface.
 3. The process of claim 1 wherein said aluminum-silicon base alloy includes as an alloying element at least one of the metals of the first group.
 4. The process of claim 1 wherein the first metal is an elongated tubular article and said aluminum-silicon base alloy is a conforming tubular article, said conforming tubular article positioned within and in intimate contact with said first tubular article for protecting the interior surface of said first tubular article.
 5. The process of claim 4 wherein an expandable plug having a substantially greater coefficient of expansion than said tubular articles is fitted into the interior of and in contact with the interior surface of said conforming tubular article, and heating said tubular articles and said plug, whereby said plug expands to a greater extent than said tubular articles to force said conforming tubular article against the interior surface of said first metal tubular article.
 6. The process of claim 5 wherein said heating is from about 380* to 660* C. at a pressure up to 10 3 torr for a period of at least 1 hour to braze said tubular members.
 7. In a process for protecting the interior surface of a first elongated tubular article of a metal selected from the group consisting of Zr, Ti, Hf, Nb, V and alloys thereof from corrosion at high temperatures by brazing the outer surface of a second tubular aluminum-silicon base alloy article to said interior surface with a brazing material selected from the group consisting of Cu, Ag, Be, Ge, Si and alloys thereof; the improvement comprising: galvanizing the outer surface of said second tubular article. 